1. Field of the invention
The present invention concerns videotex terminals such as the "Minitel" terminal used in France. It is more particularly concerned with a terminal of this kind connected to a public switched telephone network and equipped with an auto-answer device.
2. Description of the prior art
The connection between a videotex terminal and a data server via a public switched telephone network is usually set up using a subscriber telephone. The telephone is connected via the terminal to a telephone network jack. Initially the telephone is connected to the telephone subscriber line while the terminal itself is not connected to the network. A switch in the terminal performs this function. The same switch is operated to connect the terminal to the telephone line once the connection has been set up.
The public switched telephone network is accessed to set up the connection to the server by means of the telephone with the handset off-hook. The subscriber dials the number of the data center for the server or the server's own number. The call is switched through the telephone network and is sensed at a connection center which makes the connection to the server. A return tone is sent by the server to the subscriber by way of a response. The subscriber hears this tone and recognizes that it confirms that the connection to the server has been set up. The subscriber then switches the telephone line to his terminal by pressing the key provided for this purpose to operate the aforementioned switch and then hangs up the handset of his telephone.
To enable it to be detected at the receiving end the ringing signal sent over the telephone line has defined characteristics. It represents the application of a low-frequency alternating current voltage between the line wires, usually at a frequency of 50 Hz and at a level between 25 V and 96 V.
The response tone is also predefined. Its frequency is 2100 Hz.
When the terminal and the server are connected via the network data can be exchanged between the terminal and the server using the link so set up.
To enable the exchange of data between the terminal and the server the terminal incorporates a modem connected via a duplexer to a common data transmit/receive channel and a microprocessor-based processor unit connected to the modem. It further comprises an isolating transformer coupling the common transmit/receive channel to the two wires of the telephone line. A current generator connected between the terminals of the transformer winding connected to the line wires maintains a direct current on the telephone line immediately the telephone connection is set up and throughout the duration of the connection, to enable the central office to sense that the telephone line is busy.
The processor unit is connected to the data entry keyboard and to the display screen of the terminal and optionally to a printer. The modem converts digital data representing data entered at the keyboard and data displayed on the screen into analog data signals transmitted in one direction or the other on the telephone line. The duplexer transmits these signals and splits the common transmit/receive channel connected to the line into two separate transmit and receive channels connected to the modem. Bidirectional transmission of the signals on the telephone line is achieved by assigning two frequency bands for the two signal transmission directions. The frequency bands are centered on 420 Hz for the signals transmitted from the terminal to the server and 1700 Hz for those transmitted from the server to the terminal. Transmission from the terminal to the server is at a relatively lower rate of 75 bauds and transmission from the server to the terminal is at a relatively higher rate of 1200 bauds, at a low peak level in either direction of 1 V maximum.
The transformer isolating the terminal from the telephone network is matched to the characteristics of the telephone line transmitting the data signals so as to have in conjunction with the current generator (or with the transmit channel of the duplexer) an impedance matched to the defined impedance of the telephone line, which is usually 600 ohms.
Adapting a "conventional" videotex terminal of the kind described to provide an auto-answer videotex terminal entails associating with the circuits assigned to the data signals and isolated from the network by the isolating transformer circuits assigned to the ringing signals to be detected, separately isolated from the network.
Compared with a conventional videotex terminal, the cost of an auto-answer videotex terminal of this kind is increased by the cost of implementing the ringing signal detection function including the isolation specific to this function.
An object of the present invention is to provide an auto-answer videotex terminal with ringing signal detection incorporated into the terminal at reduced cost.